- Haskell-Cafe - Haskell.org

Painting logs to get a coloured tree
by oleg＠okmij.org 10 Feb '09

10 Feb '09

If I understand you correctly, the problem is to annotate an already constructed tree with arbitrary pieces of new data -- hopefully without reconstructing the tree. Perhaps the approach used in the FLOLAC type-checkers would be helpful. The `tree' was an expression in lambda-calculus to type check. The new annotations are the reconstructed types, of each node in the tree (of each sub-expression). I wanted to annotate each sub-expression with its reconstructed type -- without re-defining the data type of expressions or rebuilding the tree. The expression should stay as it was, I merely want to associate, after the fact, additional pieces of data with each node. I also wanted the code to be pure and avoid STRefs let alone StableNames and any IO. Here are the files in question: http://okmij.org/ftp/Computation/FLOLAC/TEvalNC.hs http://okmij.org/ftp/Computation/FLOLAC/TEvalNR.hs TEvalNC.hs is the ordinary type checker for the simply-typed lambda calculus with constants and the fix-point. The type reconstructor (aka, non-standard, abstract evaluator) has this type teval :: TEnv -> Term -> Typ Given the type environment and a term, it returns its inferred type (or reports an error). The file TEvalNR.hs returns not only the reconstructed type of the term but also the types of all the subterms. The latter data are returned in a `virtual' typed AST -- virtual because the original AST is not modified and the inferred types are attached to AST nodes, well, virtually. Generally speaking, after a simple modification teval could be made total: it would return reconstructed types of the subterms even if the entire term is ill-typed. That modification was one of the exercises. The intention was to model OCaml -- which, given a special flag, can dump the inferred types of all sub-expressions, even if the overall type checking failed. In Emacs and vi, one can highlight an expression or variable and see its inferred type. If the type checking failed, one can still explore what the type checker did manage to infer.

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Haskell and Java interaction
by Silviu ANDRICA 10 Feb '09

10 Feb '09

Hello, I was wondering if there is a way to call Haskell code from Java. I tried using jvm-bridge(http://sourceforge.net/projects/jvm-bridge/), but I'm stuck on building it. Thank you very much, Silviu

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lazy evaluation is not complete
by Peter Padawitz 10 Feb '09

10 Feb '09

A simplied version of Example 5-16 in Manna's classical book "Mathematical Theory of Computation": foo x = if x == 0 then 0 else foo (x-1)*foo (x+1) If run with ghci, foo 5 does not terminate, i.e., Haskell does not look for all outermost redices in parallel. Why? For efficiency reasons? It's a pity because a parallel-outermost strategy would be complete.

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Re: [Haskell-cafe] ANN: convertible (first release)
by Daniel Schüssler 10 Feb '09

10 Feb '09

Hi, On Wednesday 28 January 2009 04:30:07 John Goerzen wrote: > On Tue, Jan 27, 2009 at 09:41:30PM -0500, wren ng thornton wrote: > > I once again point out that realToFrac is *wrong* for converting from > > Float or Double. > > > > > realToFrac (1/0::Float) ::Double > > > > 3.402823669209385e38 > > Yes, I understand what you are saying and agree with you. But there > is nothing better in the standard library don't know whether you consider GHC as standard library, but if you do...: import GHC.Types import GHC.Prim -- | Double to Float d2f (D# d) = F# (double2Float# d) -- | Float to Double f2d (F# f) = D# (float2Double# f) ghci> f2d (1/0) Infinity it :: Double :) -- Daniel

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Re: [Haskell-cafe] Formal semantics for Haskell?
by gregg reynolds 09 Feb '09

09 Feb '09

Is that () or _|_? ;) Tim Newsham <newsham(a)lava.net> wrote: >null

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Re: Haskell and Java interaction
by John A. De Goes 09 Feb '09

09 Feb '09

The bridging projects are all dead and unsupported. You can still do it, but you'll have to write Haskell wrappers that use C types, then write C code that calls the wrapped Haskell code, then write JNI code to call the C code from Java. http://www.haskell.org/haskellwiki/Calling_Haskell_from_C There was a promising thesis project called LambdaVM that allowed you to compile Haskell to JVM byte codes, but it suffered the same fate as all thesis projects. Regards, John A. De Goes N-BRAIN, Inc. The Evolution of Collaboration http://www.n-brain.net | 877-376-2724 x 101 On Feb 9, 2009, at 8:56 AM, Silviu ANDRICA wrote: > Hello, > I was wondering if there is a way to call Haskell code from Java. I > tried using jvm-bridge(http://sourceforge.net/projects/jvm-bridge/), > but I'm stuck on building it. > > Thank you very much, > Silviu > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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how to use a literal string with bystring useAsCString
by minh thu 09 Feb '09

09 Feb '09

Hi, I have a Haskell source file encoded in utf-8. Inside that source, I have literal strings that I'd like to pass to a C function. withCString does the job well until I tried to use the double-quote character ". I get /usr/lib/ghc-6.10.1/ghc: `@: Bad font file format (even when using (chr 34) instead). I didn't understand the reason of this behavior (since the double quote is just ascii) but tried to use useAsCString but coudn't do it. I can ByteString.Char8.pack my string but the problem remains. I tried to use IConv but it uses UTF8.ByteString and I don't know how to make the conversion so I can use useAsCString or withCString. Any idea ? Thanks, Thu

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Control.Arrow being icky
by Louis Wasserman 09 Feb '09

09 Feb '09

In GHCi, I import Control.Arrow, and Kliesli doesn't appear: Prelude Control.Arrow> :t Kliesli <interactive>:1:0: Not in scope: data constructor `Kliesli' Prelude Control.Arrow> :browse (<<^) :: (Arrow a) => a c d -> (b -> c) -> a b d (>>^) :: (Arrow a) => a b c -> (c -> d) -> a b d class (Control.Category.Category a) => Arrow a where arr :: (b -> c) -> a b c first :: a b c -> a (b, d) (c, d) second :: a b c -> a (d, b) (d, c) (***) :: a b c -> a b' c' -> a (b, b') (c, c') (&&&) :: a b c -> a b c' -> a b (c, c') class (Arrow a) => ArrowApply a where app :: a (a b c, b) c class (Arrow a) => ArrowChoice a where left :: a b c -> a (Either b d) (Either c d) right :: a b c -> a (Either d b) (Either d c) (+++) :: a b c -> a b' c' -> a (Either b b') (Either c c') (|||) :: a b d -> a c d -> a (Either b c) d class (Arrow a) => ArrowLoop a where loop :: a (b, d) (c, d) -> a b c newtype (ArrowApply a) => ArrowMonad a b = ArrowMonad (a () b) class (ArrowZero a) => ArrowPlus a where (<+>) :: a b c -> a b c -> a b c class (Arrow a) => ArrowZero a where zeroArrow :: a b c newtype Kleisli m a b = Kleisli {runKleisli :: a -> m b} (^<<) :: (Arrow a) => (c -> d) -> a b c -> a b d (^>>) :: (Arrow a) => (b -> c) -> a c d -> a b d leftApp :: (ArrowApply a) => a b c -> a (Either b d) (Either c d) returnA :: (Arrow a) => a b b (<<<) :: (Control.Category.Category cat) => cat b c -> cat a b -> cat a c (>>>) :: (Control.Category.Category cat) => cat a b -> cat b c -> cat a c Does anybody know what's going on? Louis Wasserman wasserman.louis(a)gmail.com

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Efficient string output
by Ketil Malde 09 Feb '09

09 Feb '09

Hi, I'm currently working on a program that parses a large binary file and produces various textual outputs extracted from it. Simple enough. But: since we're talking large amounts of data, I'd like to have reasonable performance. Reading the binary file is very efficient thanks to Data.Binary. However, output is a different matter. Currently, my code looks something like: summarize :: Foo -> ByteString summarize f = let f1 = accessor f f2 = expression f : in B.concat [f1,pack "\t",pack (show f2),...] which isn't particularly elegant, and builds a temporary ByteString that usually only get passed to B.putStrLn. I can suffer the inelegance were it only fast - but this ends up taking the better part of the execution time. I tried to use lazy ByteStrings, the theory being that the components that already are (strict) ByteStrings could be recycled as chunks. I also tried to push the output down into the function (summarize :: Foo -> IO ()), but both of these were actuall slower. Since I surely can't be the first person that needs to output tab-separated text, I'd be grateful if somebody could point me in the right direction. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Formal semantics for Haskell?
by Gregg Reynolds 09 Feb '09

09 Feb '09

ML has a formal definition[1]; why not Haskell? Would this be a Good Thing, or a Waste Of Time? The Report strikes me as a hybrid of formal and informal. I guess that's not a problem so far, but there aren't that many implementations; if we had dozens, how could we verify conformance? A formal semantics would be useful, but it would also be Fun to use Category Theory notation in a language definition. Such a task would be way beyond my abilities, but I have come up with an idea for a formal semantics of IO and other non-deterministic elements of the language that I think is kind of interesting. It's inspired by Category Theory and the Z specification language. See my (brief) blog article<http://syntax.wikidot.com/blog:3> . Actually, I'm in a state of rather intense euphoria about it, so a bucket of cold water realism over my head might be a Good Thing. Then I could get some sleep instead of obsessing about category theory and Haskell. :) I propose any formal definition include the following warning, modeled on Knuth's warning about MetaFont: WARNING: Haskell can be hazardous to your other interests. Once you get hooked, you will develop intense feelings about language design; semantic models will intrude on the program texts you read. And you will perpetually be thinking of improvements to the programs that you see everywhere, including those of your own design. Thanks, gregg [1] The Definition of Standard ML (Revised); a preview is on Google Books

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